Why Some Towers Leave a Floor Open to the Wind

By Michael Stern, JDS Development Group

Somewhere above the occupied floors of a very slender tower there is a level with no walls and no glass, open to the air on all four sides. From the sidewalk it reads as a gap in the building, a band of shadow where the windows stop and then start again. It is there because of what moving air does to a tall narrow prism, and it is one of the few places in a tower where the shape of the building and the behavior of the wind are a single decision.

This is written for architects, structural engineers and developers looking at a footprint that is too small for the height they want. The question underneath it is how much of a tall building has to be given up to the air before the rest of it will hold still.

How Wind Sheds Vortices Off a Tall Tower

Air arriving at a tall building does not flow smoothly around it. It separates at the corners and rolls off each face in turn, first one side and then the other, in a rhythm rather than a steady push.

That alternation loads the tower across the wind as well as along it, and the rhythm has a rate set by the width of the building and the speed of the air. When that rate comes close to the rate at which the tower itself wants to sway, each shed vortex arrives in time with the motion already there and adds to it. The result is a larger movement rather than a stronger push, built up over minutes instead of arriving in a gust.

Vortex shedding is the part of wind engineering that architects hear about least and feel most, because its product is movement rather than damage. What fails first is the willingness of a person on an upper floor to stay in the room, and occupant comfort under a rare wind is the limit the whole lateral design is written against.

The pressures themselves come from the loading standard American engineers design buildings to, which covers the ordinary cases well and says plainly that the unusual ones need testing. A very slender residential tower is an unusual case by definition, so the standard hands the problem to a wind tunnel.

A slender tower is exposed to this in a way a broad one is not. It is light for its height, it sways slowly, and its narrow faces shed at a rate that sits uncomfortably near that slow sway. At 111 West 57th Street the governing condition is the slenderness itself, since it is the tallest and most slender residential building in the Western Hemisphere, and on a tower in that range the wind rather than gravity is what the engineering answers to.

Why an Open Floor Interrupts the Rhythm

Everything a tall building does about vortex shedding is an attempt to stop the wake from organizing itself. A tower whose plan changes as it rises sheds at a different rate at every height, so the rhythms never add up. A tower that twists does the same more gradually, and softened or notched corners weaken the separation that starts the process.

An opening through the building is the bluntest member of that family and often the most effective. Air that passes through the tower rather than around it spoils the pressure difference between the two faces, which is the thing the alternating shedding depends on. The wake downstream of an open level never organizes, and the floors above and below shed into air already disturbed.

An open floor is an aerodynamic device before it is a place for equipment.

None of this is new and none of it is proprietary. Openings, tapers, twists and softened corners have been studied and published for decades by the international wind engineering community, and any competent tunnel tests several of them on the same model in the same week. What is specific to a tower is which of them the site and the plan will tolerate.

This is also why the open level tends to appear near the top of a tower rather than at its base. The sway of a tall building is largest at the top, the wind is strongest there, and an interruption placed high has the longest lever arm on the motion it is meant to spoil.

Where the Open Level Goes in the Section

The position is read off the wind tunnel rather than chosen from a drawing. A scale model of the tower and of the buildings around it is tested across the full circle of wind directions, and the instruments report pressures on the facade and accelerations at the top.

What comes back is a map of behavior by direction rather than a single number, because the neighbors shelter the tower from some winds and funnel others onto it. The directions that produce the worst crosswind response are the ones the design has to answer, and the height at which the response is built up tells the team where an interruption would do the most good. The wind engineer and the structural engineer of record read that together and decide whether the tower needs porosity, more damping, a different plan shape, or some combination of the three.

The tunnel is a model of the world as well as of the building, which is where its weakness lies. Neighbors that have not been built yet are guessed at or left out, and a tower sheltered on the day it opens can be exposed a decade later. Full scale measurement on finished buildings, of the kind national engineering laboratories keep doing, is how the profession checks that the small models tell the truth.

Only then does the level land in the section, and by that point it is usually welcome for a second reason. Mechanical plant wants to be high in the building, it has no need of daylight, and a floor that was going to be full of equipment is the easiest floor in the tower to open to the weather.

What an Open Floor Costs the Building

It costs a floor. On a site where the floor area was assembled out of more than one lot, giving a whole level back to the air is a real decision and it is made once, early, and for good.

It also complicates everything that has to get past it. Every riser passes through a zone that is outdoors, so pipework is traced and insulated for weather it would never otherwise see, electrical work is detailed for wind driven rain, and the elevator shafts through that level are sealed and pressurized. Facade access is harder as well, because the rig on the roof has to reach the faces above and below an opening that breaks the track.

There is a middle version, which is to screen the opening with louvers so the level reads as part of the facade while the air still moves through it. That buys the elevation back at the cost of turning the screen into a structural element, since every blade and every support now takes the full pressure at the most exposed height of the tower. Whether a screened opening does the same aerodynamic work as a bare one is the kind of question handed to an independent structural review before the drawings are final.

A competent engineer can push back on all of this, and some do. The argument against porosity is that a tuned damper and a well shaped plan will deliver the same occupant comfort without surrendering a floor, and that a wind tunnel result showing benefit in the worst direction is thin ground on which to remove a level of the building. My answer is that an opening changes the flow itself rather than absorbing what the flow produces, so it keeps working when a mechanical system is offline for service, and it does not care whether anyone remembered to retune it after ten years.

The honest compromise is that nobody can tell you in advance how much of the improvement came from the opening. Shaping, corner treatment, damping and porosity are tested as a package and they interact, so the open floor is bought as part of an answer rather than proved on its own. I have never seen that resolved cleanly and I do not expect to.

The people who decide this are the wind engineer who runs the tunnel study, the structural engineer of record who owns the response of the building, and the facade consultant who has to make the opening weathertight at its edges. A developer contributes by asking for the study early enough that its answer can still change the plan, which means before the massing is fixed rather than after.

A tower that leaves a floor open is telling you, in the only language a building has, exactly where the air was winning.

Michael Stern is the founder and chief executive officer of JDS Development Group.